The use of composite materials in the manufacture of aircraft and other lightweight structures has increased steadily since the introduction of such materials. Composite materials have a high strength-to-weight ratio and stiffness, making them attractive in the design of lightweight structures. One drawback to using composite materials in the past has been high fabrication costs. It has been difficult to produce composite parts for the same cost as comparable metal parts. The cost differential is especially notable in large-scale parts or parts having abrupt or complex contours.
One of the largest contributors to composite structure costs is the amount of manual labor required during fabrication. Composite parts having abrupt or complex contours must be formed by manually laying up individual layers of composite material on a lay-up mandrel. Larger parts having more gradual contours, for example, large wing skins, may be fabricated using automated tape laying machines.
Automated tape laying machines have a robotic arm that places individual layers of composite prepreg on a mandrel to form a composite part. Automated tape laying machines can place individual layers of prepreg at a high rate, thus possibly reducing fabrication time and cost. The mechanics of the automated tape laying machine limits its ability to place layers of composite prepreg over some geometries. Automated tape laying machines generally cannot place composite prepreg around abrupt curvatures or tight radiuses of curvature.
A common composite support structure used in the construction of aircraft and other light-weight structures is the "I"-beam or "T"-stringer. Due to abrupt curvatures, I-beams, and other beam-type composite structures are generally formed by manually placing layers of composite material over a lay-up mandrel. An automated cutting machine is used to cut each layer of composite prepreg to the proper shape. The individual layers of prepreg are then placed on two separate lay-up mandrels by hand. Once properly positioned, each layer of prepreg is manually formed to the exterior contours of the lay-up mandrel to form two "C-channels."
The C-channels and lay-up mandrels are then rotated so that the C-channels can be joined together along their vertical webs to form an I-beam. After joining, a triangular composite radius filler is placed in the triangular recesses formed in the center of the top and bottom flanges during joining. Top and bottom composite reinforcement layers are then manually placed on the top and bottom of the I-beam. The resulting I-beam is then bagged and autoclave cured.
The entire I-beam fabrication process is labor intensive and time consuming. In addition to the labor required to place the composite material, labor is also required to manipulate the lay-up mandrels. Generally, lay-up mandrels are quite large and heavy, making them difficult and dangerous to maneuver. After the two C-channels are formed, a crane or similar handling device is used to engage the individual lay-up mandrels and transport them to an assembly table. The mandrels are then manually maneuvered, rotated, and aligned using levers, etc. The two channels are then bonded together using a vacuum membrane in a laborious, time-consuming process. Once bonded, a triangular radius filler is added to the top. The lay-up mandrels are then rotated 180.degree. so that a triangular radius filler may be applied to the bottom of the I-beam. Finally, the resulting I-beam and lay-up mandrels must be placed on a curing mandrel for vacuum bagging and subsequent curing.
In addition to being costly, the extensive use of manual labor during the fabrication process produces quality control problems. Individual layers of composite prepreg are often incorrectly positioned. Extensive manual handling during fabrication also increases the likelihood of foreign materials being introduced into the completed part. Incorrectly placed layers of composite and foreign material are large contributors to overall part rejection rates.
Thus, a need exists for methods and apparatus that reduce the amount of manual labor required during the fabrication of structures, particularly large structures, from composite materials. The present invention is directed to meeting this need.